In considering the expansion of existing outside telephone plant facilities or new outside plant installations the economics of two alternatives are usually weighed. One is the use of non-carrier, physical pairs to provide service for new subscribers. The other is the use of multi-channel station carrier equipment to furnish the needed service.
The cost of cable in multi-channel station carrier systems is considerably less than the cable costs attendant with the use of physical pairs because of the more efficient use of cable in the carrier systems. At some point, however, the cost of known commercially available multi-channel station carrier equipment outweighs the more efficient use of cable, thus making it uneconomical to use such station carrier systems on relatively short transmission circuits. Cost therefore becomes one of the foremost considerations in the design of multi-channel station carrier equipment in order to make station carrier an economically attractive alternative to physical pairs. Additionally, there is a continuing need to improve performance of station carrier equipment.
To achieve acceptable performance, automatic carrier level coordination control is required in multi-channel station carrier systems and adds significantly to the cost of the electronic equipment. Carrier level coordination control involves the coordination of carrier levels between two or more carrier systems operating at corresponding frequencies and having their transmission lines in the same multiconductor cable or in otherwise close proximity to each other. The carrier signal power levels in the system are so adjusted that at any point along the cable carrier signals of like frequencies will be at least approximately at the same power level to minimize crosstalk due to power differentials.
In the past, it was the general custom to equip each subscriber terminal unit or circuit in the multi-channel carrier system with a separate automatic carrier level coordination control circuit. Each of these prior automatic coordination control circuits individually performs a complete automatic control function in the sense that it measures the transmission line length over which the subscriber carrier signal is to be transmitted and then adjusts the subscriber transmit carrier power level as a function of the measured transmission line length and a function of the carrier signal frequency. One example of such an automatic coordination control circuit is described in U.S. Pat. No. 3,475,561 which issued to L. Q. Krasin and C. E. Greene on Oct. 28, 1969.